Aliphatic polyesters such as polyglycolic acid and polylactic acid are degraded by microorganisms or enzymes present in the natural world such as in the ground or the sea and have therefore attracted attention as biodegradable polymer materials with a small environmental burden. In addition to the biodegradability, these aliphatic polyesters have hydrolyzability and use of the aliphatic polyesters in various fields has been actively investigated in recent years.
Among aliphatic polyesters, polyglycolic acid (hereinafter also called “PGA”) has excellent gas barrier properties and excellent heat resistance and/or mechanical strength. Therefore, the PGA is expected to be a packaging material for, for example, food that is susceptible to oxidative degradation and also a packaging material that is easily composted and has a small environmental burden. Furthermore, since the PGA has biodegradability and bioabsorbability, the PGA has been used as fibers in a medical field or the like.
In order to obtain petroleum and natural gas, oil wells and gas wells are drilled. Such drilling operations include the process of fracturing which increases the production of the petroleum and/or natural gas by boring a wellbore using a drill while mud water is circulated and then injecting fracturing fluid into a subterranean formation to create fractures. In addition, a treatment of oil wells and gas wells after drilling and completion operations using a slurry containing a degradable material has been proposed (e.g. see U.S. Pat. No. 7,775,278B specification (Patent Document 1)). Furthermore, WO/2007/066254 (Patent Document 2) discloses polyesters such as polylactic acid and polyglycolic acid acting as a degradable material constituting the fracturing fluid.
Note that, the present applicant has discovered that a dispersion liquid in which a particular polyglycolic acid resin in a finely divided solid state is dispersed in an aqueous medium is suitable for drilling or completion of oil and gas recovery, and has filed a PCT application (PCT/JP2011/73646).